The invention relates to an apparatus for the continuous feeding of charge material to a melting crucible in the pulling of monocrystals in a closable container with a lock valve and a discharge tube leading into the crucible.
In the semiconductor industry, silicon substrates are desirable which have a uniform quality throughout. In the production of the melt with the known apparatus, a varying distribution of the dopants occurs due to varying temperatures in the melt. Metals produced in this manner have different properties and resistance coefficients, which is not desirable. For this reason the crucible in the prior-art method for the drawing of monocrystals has been run down to 50% and then refilled with fresh granules or undoped silicon. The process is thus frequently interrupted so that such apparatus have not been able to operate economically. Consequently attempts have been made to feed fresh granules continuously during the working process. However, in the known system problems are involved in the feeding of granules and the dust that it raises, plus a possible disturbance of the temperature equilibrium by the directly doped granules.
U.S. Pat. No. 5,180,562 discloses a crucible to hold the melt, a heating system with a pulling means above the melt, and a cover with an opening. The heater required for this purpose is of a ring-like construction. The granules are melted by a separate heater in the external part of a divided crucible. With this known apparatus a variety of processes have to be performed in a very tight space. For one thing, the conditions for melting must be established, and on the other hand the crystal in the inner area must be pulled without disturbance, i.e., without vibration and thermal shock.